Layers_by_spxl

I like the idea of having artificial "construction lines" optionally
appear in an image, similar to showing a "wireframe" on a 3D model. This
example uses two layers of construction lines: one behind the objects, and
one in front of the objects. The level of detail can also be adjusted; in
this case, the crosshairs to the edge of the window are at a higher
level (not to be confused with higher layer!) than the small "+" to mark
a circle's centre and a circle's outline.

The base Shape class includes details for presentation (fill colour,
edge colour, edge weight, etc) as well as position and animation (in this
case simple velocity with acceleration), including drawing a "trace" of
the shape's location. The motion calculations aren't "reality-accurate",
but they are time based, not frame based. To demonstrate this, you can
increase and decrease the sketch's frame rate and see that the objects
seem to move in similar trajectories whether the updates are regular
(eg at 32 fps) or infrequent (eg at 1 fps). Time is "stopped" (or more
like skipped) when the sketch is paused.

Another neat trick implemented is seamless wrapping of objects at the
edges of the window. This is achieved by drawing an object multiple times,
if required, offset by the window width and/or height. A similar trick is
used to wrap the trace line generated for the shape.